At the top of the Firefox window, click on the Firefox button, go over to the History menu and select Clear Recent History....
   For Windows XP: At the top of the Firefox window, click on the Tools menu and select Clear Recent History....
   In the Time Range to clear: drop-down, select Everything.
   Click the arrow next to Details to display the list of items that can be cleared.
   Select both Cookies and Cache.
   Click Clear Now. 

Did you try clearing your cache and cookies? At the top of the Firefox window, click on the Firefox button, go over to the History menu and select Clear Recent History.... For Windows XP: At the top of the Firefox window, click on the Tools menu and select Clear Recent History.... In the Time Range to clear: drop-down, select Everything. Click the arrow next to Details to display the list of items that can be cleared. Select both Cookies and Cache. Click Clear Now. Please see [[Problems using Facebook in Firefox]] for more information